Weed Science 2009 57:271–280 Adaptive Responses of Field-Grown Common Lambsquarters (Chenopodium album) to Variable Light Qualityand QuantityEnvironments Greta G. Gramig and David E. Stoltenberg* Field experiments were conducted to determine whether exposure to reduced red : far-red light ratios (R : FR) typical of crop–weed environments was associated with adaptive changes in morphology, productivity, and fecundity of common lambsquarters. Plants were grown in reduced or ambient R : FR environments (both in full sunlight) until initiation of flowering, after which plants were grown in full sunlight or partial shade. At initiation of flowering, plants that had been exposed to reduced R : FR exhibited greater specific leaf area, stem elongation, main stem leaf area, specific stem length, and main stem mass compared with plants exposed to ambient R : FR. However, biomass allocation to stems, leaves, and roots did not differ between vegetative-stage R : FR treatments. At the end of flowering, morphology and productivity of plants exposed to partial shade did not differ between vegetative-stage R : FR treatments. In contrast, plants exposed to full sunlight during flowering after exposure to reduced R : FR during the vegetative stage had less total plant mass, less total leaf area, greater stem elongation, greater specific stem length, and a greater ratio of main stem to total stem mass compared with plants exposed to ambient R : FR during the vegetative stage. At physiological maturity, plants exposed to reduced R : FR during the vegetative stage and to partial shade during the reproductive stage had less total seed mass and fewer seeds compared with plants exposed to ambient R : FR during the vegetative stage and to partial shade during the reproductive stage. Fecundity of plants exposed to full sunlight during the reproductive stage did not differ between vegetative-stage R : FR treatments. These results indicate that exposure of common lambsquarters to reduced R : FR during the vegetative stage was maladaptive at later stages of growth in competitive environments, and suggest that interactions of light quality and quantity are important determinants of common lambsquarters fecundity. Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL. Key words: Competition, weed–crop interactions, light quality, red : far-red ratio, modeling. Light serves not only as a resource for plants, but also as a in leaf anatomy and morphology, alterations in leaf and shoot source of many cues that trigger plant responses to the orientation, increased apical dominance, and early flowering surrounding environment (Aphalo and Ballare´ 1995; Ballare´ (Ballare´ and Casal 2000; Kasperbauer 2000). 1999). Previous research has shown that many important Numerous studies conducted in controlled environments responses of plants to variable light environments are and in the field have documented plant morphological mediated by light wavelength-sensitive photoreceptors. Plants changes in association with changes in R : FR. Studies have signal-transducing photoreceptors belonging to three involving dicotyledonous species have demonstrated that major classes: red (R) and far-red (FR) wavelength-sensitive plants exposed to reduced R : FR exhibit greater internode phytochromes, UV-A/blue wavelength-sensitive crypto- length (Board 2001; Causin and Wulff 2003), greater plant chromes, and phototropins (Ballare´ 1999; Franklin and height (Mahoney and Swanton 2008; Rajcan et al. 2002), Whitelam 2004). Of these, the phytochrome class of reduced ratio of leaf mass to stem mass (LSR) (Causin and photoreceptors has been the most widely studied at molecular, Wulff 2003), greater specific leaf area (SLA) (Causin and cellular, and whole-plant levels (Franklin and Whitelam Wulff 2003), increased branch orientation away from nearby 2004). Many studies involving Arabidopsis thaliana mutants neighbors (Novoplansky 1990, 1991), greater leaf area have characterized the genetic and biochemical basis of plant (Heraut-Bron et al. 2001), reduced leaf number and more responses to changes in the spectral composition of light. steeply inclined leaves (van Hinsberg and van Tienderen These and other studies have shown that phytochromes play a 1997), and reduced total plant mass (Causin and Wulff 2003) critical role in mediating morphological responses of plants to when compared with plants exposed to ambient R : FR. changes in the ratio of R to FR wavelengths (R : FR) (Casal et Few studies have determined the effects of R : FR-mediated al. 1998; Franklin and Whitelam 2004; Weinig 2000). alterations in weed and crop morphology on competitive Because green plants preferentially absorb light in the R and ability under field conditions (Ballare´ and Casal 2000). Such blue regions of the visible spectrum, light microenvironments responses could have important consequences for modeling adjacent to plants tend to be depleted in R wavelengths and competition for light in crop–weed communities (Rajcan and enriched in FR wavelengths (Ballare´ 1999; Smith 1982). Swanton 2001; Rajcan et al. 2004). Most plant species, in Therefore, reduced R : FR, whether occurring alone or particular weed species, are characterized by a high degree of together with reduced photosynthetically active radiation morphological plasticity in changing light environments. A (PAR), is a reliable indicator of impending or actual shade better understanding of this plasticity could help explain the (Aphalo and Ballare´ 1995; Smith 1982). Well-documented underlying basis for phenomena such as critical periods for phytochrome-mediated responses of plants to reduced R : FR weed removal and economic thresholds for weed control that include stem elongation, suppression of branching or tillering, are only described empirically (Holt 1995; Rajcan and shifts in biomass allocation to stems, roots, and leaves, changes Swanton 2001). Also, most process-based models of plant competition, which are primarily driven by resource avail- DOI: 1614/WS-08-120.1 ability and use, tend to neglect plant morphological plasticity * Department of Agronomy, University of Wisconsin, 1575 Linden Drive, Madison, WI 53706. Current address of first author: Department of Plant as influenced by the light environment (Aphalo et al. 1999). Sciences, North Dakota State University, 166 Loftsgard Hall, Fargo, ND 58102. Furthermore, the lack of process-based descriptions of Corresponding author’s E-mail: [email protected] biomass allocation is a fundamental weakness inherent in Gramig and Stoltenberg: Common lambsquarters adaptive responses to light N 271 most simulation models of plant growth and development by seedbed preparation with a soil finisher in early May. (Grace 1991). Glyphosate- and European corn borer [Ostrinia nubilalis Greater understanding of plant morphological responses to (Hu¨bner)]-resistant ‘DeKalb 50-20 RR2/YGCB’ corn seed the light environment could improve methods of biomass was planted at a rate of 80,000 seeds ha21 in 76-cm east–west partitioning in models describing weed growth and compe- oriented rows on May 7, 2004 and May 10, 2005. Tefluthrin tition with crop plants (Aphalo and Ballare´ 1995; Grace 1991; (0.15 kg ai ha21) was applied in-furrow at planting to protect Rajcan and Swanton 2001). For instance, simulation of corn from corn rootworm (Diabrotica spp.) infestation. jimsonweed (Datura stramonium L.) growth and competitive Before corn emergence, 1.85 kg ai ha21 S-metolachlor, effect on corn (Zea mays L.) was greatly improved when the 0.17 kg ai ha21 mesotrione, and 0.69 kg ai ha21 simazine model INTERCOM was modified to include plasticity of were applied for weed control. height and SLA that occurred under competitive conditions On June 15, 2004 and June 14, 2005 (at approximately the for light (Cavero et al. 2000). Conversely, these adjustments fifth-leaf stage of corn), common lambsquarters seeds were did not improve simulations when the model was parameter- planted in 11.4-L plastic pots (156 pots total) filled with a ized to describe growth in monocultures of either species. 50 : 50 sand : silt loam mixture that had been screened to Most plant growth simulation models describe competitive remove large soil aggregates, rocks, and organic debris. The interactions that occur only when resources (such as light) sand was sharp coarse quartz sand and the silt loam was as become limiting. If responses occurring before the onset of described above. The seeds had been hand harvested in the fall resource limitation are important to outcomes of competition, of 2003 from plants located in fields with a history of then characterization of these responses might improve continuous corn production at the UW-AARS. The seeds process-based models of crop–weed competition. Previous were stored at 25 C. The same seed source was used in 2004 studies have typically focused on early-vegetative-stage plant and 2005 experiments. Pots were placed 1.8 m apart responses to altered R : FR but have seldom addressed the equidistant between two rows of corn (reduced R : FR implications of these responses for long-term competitive environment) or 1.8 m apart in an open (crop-free) field ability through the reproductive stage to physiological area (ambient R : FR environment). The experimental area for maturity. It is important to know not only whether plastic each R : FR environment measured 35 m (north–south) by growth responses to altered R : FR occur early in plant 32 m (east–west). Pots were placed on plastic saucers so roots development